libiscsiutil: Fix header to work with C++

Reviewed by:	asomers
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D49137

(cherry picked from commit e94d3f65b9)

lib/libiscsiutil/libiscsiutil.h

@@ -95,6 +95,8 @@ struct rchap {
 	size_t		rchap_challenge_len;
 };
 
+__BEGIN_DECLS
+
 struct chap		*chap_new(void);
 char			*chap_get_id(const struct chap *chap);
 char			*chap_get_challenge(const struct chap *chap);
@@ -174,4 +176,6 @@ void			log_debugx(const char *, ...) __printflike(1, 2);
 
 char			*checked_strdup(const char *);
 
+__END_DECLS
+
 #endif /* !__LIBISCSIUTIL_H__ */